The present invention relates to a radial antenna and a plasma device using it.
In the manufacture of a semiconductor device, plasma devices are used often to perform processes such as formation of an oxide film, crystal growth of a semiconductor layer, etching, and ashing. Among the plasma devices, a microwave plasma device is available which produces a high-density plasma by introducing a microwave into a processing vessel through a radial antenna. As a characteristic feature of the microwave plasma device, it has wide applications because it can stably produce a plasma even if the pressure is comparatively low.
FIG. 7 includes views showing the arrangement of an example of a radial antenna conventionally used in the microwave plasma device, and the distribution of its electric field radiation. FIG. 7(a) is a conceptual view showing the radiation surface of the radial antenna, FIG. 7(b) is a sectional view taken along the line VIIb-VIIb′ of FIG. 7(a), and FIG. 7(c) is a conceptual view showing the distribution of the electric field radiated by the radial antenna. In FIG. 7(c), the axis of abscissa represents the distance from the center of the radial antenna in the radial direction, and the axis of ordinate represents the strength of the electric field radiated from the radial antenna. FIG. 8 is a view showing the shape of a slot formed in the radiation surface of the radial antenna shown in FIG. 7.
As shown in FIG. 7(b), a radial antenna 230 conventionally used in the plasma device is formed of two parallel conductive plates 231 and 232 which form a radial waveguide 233, and a ring member 234 which connects the peripheral edges of the conductive plates 231 and 232. A microwave inlet 235 is formed at the center of the conductive plate 232 to introduce a microwave from a microwave generator (not shown). The conductive plate 231 also has a large number of slots 236 to radiate the microwave propagating in the radial waveguide 233 to a processing vessel (not shown). When the influence on the electromagnetic field in the radial waveguide 233 is considered, the smaller a width W2 of each slot 236, the better. If, however, the width W2 is excessively small, it may cause abnormal discharge. Thus, the width W2 is usually set to about 2 mm (W2≦λg/4 where λg is the wavelength of the microwave in the radial waveguide 233).
The microwave introduced from the microwave inlet 235 propagates radially from the center toward the peripheral portion of the radial waveguide 233. As the microwave is radiated little by little from the large number of slots 236, the power density in the radial waveguide 233 gradually decreases toward the peripheral portion of the radial waveguide 233. The electric field radiation efficiency of the slots 236 gradually increases as their length or slot length L2 increases from 0 (zero), and reaches the maximum when the slot length L2 corresponds to λg/2.
Under these conditions, in order to obtain the radiated electric field distribution as shown in, e.g., FIG. 7(c), conventionally, the slot length L2 was adjusted, so the radiated electric field strength was controlled. More specifically, the further away from the center of the conductive plate 231, the larger the slot length L2, as shown in FIG. 7(a), so the slot length L2 at the peripheral portion where the power density was small was set close to a length corresponding to λg/2, thus realizing the radiated electric field distribution as shown in FIG. 7(c).
When, hover, W2=λg/2 where the electric field radiation efficiency of the slot 236 becomes maximum, the microwave resonates. Particularly, when the width W2 of the slot 236 is as small as 2 mm, abnormal discharge is induced. When this discharge heats the portion around the slot 236, the surrounding portion of the slot 236 is distorted, or starts to melt.